Aerobic degradation of 2,4,6-trinitrophenol by Proteus sp. strain OSES2 obtained from an explosive contaminated tropical soil

Abstract

A 2,4,6-trinitrophenol (TNP) degrading bacterial strain isolated from a site polluted with explosives was identified as Proteus sp. strain OSES2 via 16S rRNA gene sequencing. Metabolic investigation showed that the organism grew exponentially on 100 mg l⁻¹ of TNP as a source of carbon, nitrogen, and energy. In addition, the growth of the organism was sustainable on 3-nitrotoluene, 2,4-dinitrotoluene, 2,4,6-trinitrotoluene, 4-nitrophenol, methyl-3-nitrobenzoate, 4-nitroaniline, aniline and nitrobenzene. Strain OSES2 was able to utilize TNP within a concentration range of 100 mg l⁻¹ to 500 mg l⁻¹. The specific growth rate and degradation rates on TNP were 0.01043 h⁻¹ and 0.01766 mg l⁻¹ h⁻¹ respectively. Effective degradation of TNP in a chemically defined medium was evident with a gradual reduction in the concentration of TNP concomitant with an increase in cell density as well as the substantial release of ammonium (NH₄⁺), nitrite (NO₂⁻), and nitrate (NO₃⁻) as metabolites in 96 h. Degradation competence of the organism was enhanced in the presence of starch and acetate. On starch-supplemented TNP, the highest specific growth rate and degradation rates were 0.02634 h⁻¹ and 0.04458 mg l⁻¹ h⁻¹, respectively, while the corresponding values on acetate were 0.02341 h⁻¹ and 0.02811 mg l⁻¹ h⁻¹. However, amendment with nitrogen sources yielded no substantial improvement in degradation. TNP was utilized optimally at pH 7 to 9 and within the temperature range of 30 °C to 37 °C. The enzyme hydride transferase II [HTII], encoded by the npdI gene which is the first step involved in the TNP degradation pathway, was readily expressed by the isolate thus suggesting that substrate was utilized through the classical metabolic pathway.